Polybenzimidazoles are a known class of heterocyclic polymers. Their preparation and description are well documented in the literature. For example, see U.S. Pat. Nos. 2,895,948 and 3,174,947 and Vogel et al, J. Polymer Science, Vol. 50, pp. 511-539 (1961).
Polybenzimidazoles are characterized by a high degree of thermal stability. Moreover, they may be shaped to form fibers, films, and other articles of wide utility, such as where great resistance to degradation by heat, hydrolytic, and oxidizing media are necessary.
It has also been documented that solutions of polybenzimidazoles cannot be stored for long periods of time without "phasing out", i.e., a separation of the solution into two phases containing greater and lesser concentrations of polybenzimidazole. See U.S. Pat. Nos. 3,502,606 and 4,321,182.
U.S. Pat. No. 3,502,606 discloses that minor amounts of lithium chloride, zinc chloride, N-methyl morpholine, triethyl amine, or triethanol amine, when added to polybenzimidazole solutions, act as a stabilizer that prevents or retards "phasing out".
U.S. Pat. No. 4,321,182 discloses that minor amounts of an organo-lithium compound not only inhibits "phasing out" but also eliminates the possibility of stress corrosion cracking that arises from the use of halide based stabilizers, such as lithium chloride. In U.S. Pat. No. 4,321,182, the organo-lithium compounds are selected from the group consisting of RCO.sub.2 Li, RSO.sub.3 Li, ROSO.sub.3 Li, and mixtures of any two or all three of the foregoing wherein R is a hydrocarbon radical having 1 to 50 carbon atoms. Representative lithium salts of monocarboxylic acids are lithium formates, lithium acetate, lithium propionate, lithium butyrate, lithium isobutyrate, lithium valerate, lithium isovalerate, lithium caproate, lithium laurate, lithium cetylate, lithium stearate, etc. Representative lithium hydrocarbon sulfonates are lithium lauryl sulfonate, lithium cetyl sulfonate, etc. Representative lithium hydrocarbon sulfates are lithium lauryl sulfate, lithium cetyl sulfate, etc. The particularly preferred organo-lithium compound is lithium stearate.
The solutions of polybenzimidazole described in U.S. Pat. Nos. 3,502,606 and 4,321,182 are typically used to form shaped articles, e.g., fibers and films. Such solutions typically contained between 10-40 percent by weight polybenzimidazole. Polybenzimidazole solutions of higher concentration, e.g., up to 50 percent by weight, may be used in prepreg or laminate application, i.e., for the construction of composite materials.